1. Field of the Invention
This invention relates to a digital signal encoder for encoding an input digital signal.
2. Description of the Prior Art
In efficient coding of audio or sound signals, etc., there is known a coding technology based on bit allocation (bit assignment) in which an audio or sound input signal, etc. is divided into signal components of a plurality of channels on a time base or a frequency base, and the number of bits is adaptively allocated among respective channels. For example, for the coding technology based on the bit allocation of an audio signal, etc., there are coding technologies such as band division coding (sub-band coding: SBC) in which an audio signal, etc. is divided into signal components in a plurality of frequency bands and coding thereof is separately carried out, so called adaptive transform coding (ATC) for transforming (orthogonal-transforming) a signal on the time base to a signal on the frequency base to thereby divide that signal into signal components in a plurality of frequency bands to adaptively carry out coding every respective bands, so called adaptive predictive coding with adaptive bit allocation (APC-AB) in which the above-mentioned SBC and the so called adaptive predictive coding (APC) are combined to divide a signal into signal components in a plurality of bands and to transform the signal components in respective bands to those in a base band (low frequency band) and thereafter to carry out a linear predictive analysis of a plurality of degrees, to perform predictive coding, and the like.
In recent years, efficient coding techniques in which the so called masking effect in the human hearing sense characteristic is taken into consideration have been frequently employed. The masking effect is a phenomenon in which a signal is masked by another signal, so it cannot be heard. Instances of the masking effect are the masking effect in an audio signal on the time base and the masking effect in a signal on the frequency base.
The masking effect on the frequency base is the effect that a signal component in one frequency band is masked by a signal component in another frequency band, so sounds in the first frequency band cannot be heard. Instances of the masking effect on the time base are the temporal masking effect and the simultaneous masking effect. The simultaneous masking effect is the effect that a small amplitude sound (or noise) occurring simultaneously with a loud, that is, large amplitude sound is masked by the loud sound, so it cannot be heard. Further, the temporal masking effect is the effect that a small amplitude sound (noise) temporally preceding and succeeding a loud sound is masked by the loud sound, so it cannot be heard. The masking backward in time of the loud sound is called a forward masking, and the masking forward in time of the loud sound is called a backward masking. Further, in the temporal masking, with respect to the human hearing sense characteristic, the effect of the forward masking persists for a long time (e.g., about 100 msec.), whereas the effect of the backward masking is effective for only a short time (e.g., about 5 msec.). In addition, the level of the masking effect (masking quantity) is about 20 dB in the case of the forward masking, and is about 30 dB in the case of the backward masking. As stated above, since sound at a portion subjected to masking cannot be heard, even if the number of bits allocated for quantization of a signal component at the portion subjected to masking is reduced during quantization of an audio signal, deterioration in sound quality can be minimized.
Meanwhile, in the above-mentioned efficient coding, it is desirable to allow the bit compression ratio to be higher (reduce the bit rate further). However, generally, in the efficient coding to carry out bit compression by masking use of the masking effect as described above, only one of the masking effect in signals on the frequency base and the masking effect in signals on the time base has been limitatively utilized. Namely, both of the masking effects have not been effectively utilized together.
In addition, in the case of reducing the bit rate in consideration of the masking effect with respect to signals on the time base, it is desirable for effectively utilizing the masking effect on the time base to allow the processing time block length to be long. However, if the processing time block length is long, real time processing becomes difficult.